Various passenger protection devices such as an airbag and a seatbelt pretensioner are equipped in vehicles recently. The passenger protection system including such a passenger protection device includes, as shown in FIG. 1, front sensors 11a, 11b mounted at both front left and right sides of a vehicle 10, safety sensors 13a, 13b mounted at a front or rear passenger seats in a passenger compartment, and a plurality of sensors (first to fourth sensors) 15a to 15d, 16a to 16d mounted at both left and right sides of the vehicle 10. These sensors are connected to an electronic control unit (ECU) 18, thus forming a communications network. Each of the sensors 11a, 11b, 13a, 13b, 15a to 15d, 16a to 16d detects travel speed or collision of the vehicle, and the ECU 18 activates airbags (not shown) based on the detected travel speed or collision. Each sensor is formed in an integrated circuit (IC).
In this communications network, as shown in FIG. 2, the sensors 15a to 15d, 16a to 16d have respective bus switches 26a to 26d in the inside parts and connected to the ECU 18 through buses. These switches 26a to 26d are closed by initialization of setting addresses from the sensor 15a closest to the ECU 18, when electric power is supplied in the vehicle 10.
Specifically, the bus switch of the first sensor 15a, which is closest to the ECU 18, is set with a first address and closed to connect the second sensor 15b to the ECU 18. After setting a second address to the second sensor 15b by the ECU 18, the bus switch 26b is closed to connect the third sensor 15c to the ECU 18. After setting a third address to the third sensor 15c by the ECU 18, the bus switch 26c is closed to connect the fourth sensor 15d to the ECU 18. The initialization is performed in this order. Each sensor 15a to 15d is configured to transmit a notification to the ECU 18 after setting the address in itself.
JP 2007-215102A (U.S. Pat. No. 7,539,804) discloses a similar conventional communications network, in which an ECU communicates with sensors by setting respective addresses as described above.
According to the conventional communications networks, the bus switch 26a to 26d need be provided within the IC chip of each sensor 15a to 15d to be connected to the ECU 18. As a result, each sensor 15a to 15d becomes large in size and expensive in cost.
Further, each bus switch 26a to 26d has its impedance, which causes a voltage drop. Therefore, the voltage drop at the last-stage sensor, for example sensor 15d, becomes large, as the number of sensors 15a to 15d connected in series to the ECU 18 increases.
In addition, the impedance of the bus switch 26a to 26d is not matched between the power supply side 21 and the ground side 22, and hence noises are generated.